Dewatering process

ABSTRACT

A formaldehyde-containing product is separated from a formalin solution containing formaldehyde, water and methanol by distilling the solution in the presence of a water entraining compound, especially methyl propionate or methyl methacrylate. The product contains substantially less water than in the solution and may be used, for example, in a further process which requires a source of formaldehyde containing a relatively low level of water, for example, the catalyzed reaction of methyl propionate with formaldehyde and methanol to produce methyl methacrylate.

This application is the national phase of international applicationPCT/GB99/01724 filed Jun. 1, 1999 which designated the US was publishedin the English language.

The present invention relates to a process for the removal of water fromsolutions containing formaldehyde for the production of formaldehyde.

Formaldehyde is a commodity chemical that is conveniently produced andtransported in the form of formalin solutions. Formalin solutionscontain typically between 30% and 60% formaldehyde, the balance of thesolution being mostly water, usually with some methanol present. Theformaldehyde is predominantly present as complexes with water ormethanol in the form of glycols of hemiformals. There are a number ofmethods described in the patent literature for the dehydration offormaldehyde solutions with the object of producing a dry monomericformaldehyde stream. For example U.S. Pat. No. 4,962,235 describes thepurification of a formaldehyde/water/methanol mixture by distilling inthe presence of a polyalkylene oxide such that formaldehyde vapour isproduced at the top of the column and polyalkylene oxide, water andmethanol are removed from the bottom of the column.

NL-A-6814946 describes a process for recovering formaldehyde fromaqueous formaldehyde streams by contacting the stream with C₆-C₁₀aliphatic alcohol which reacts with the formaldehyde to form ahemiformal which can be separated from the water and subsequentlydissociated back to the alcohol and formaldehyde components.

U.S. Pat. No. 3,174,912 describes a process for the removal of water andcoloured organic impurities from a dilute aqueous formaldehyde mixtureby distillation in the presence of acetone. A mixed stream containingacetone and formaldehyde and a small amount of water is removed from thetop of the column and separated by partial condensation into a streamcontaining a relatively small amount of formaldehyde in acetone and astream containing a larger proportion of formaldehyde in acetone.

It is, however, desirable to avoid the introduction of additionalchemical compounds into a process which requires purified formaldehydebecause the additional compound may need to be removed from the processat a subsequent stage.

One process in which formaldehyde is used is that for the production ofmethyl methacrylate by the reaction of methyl propionate withformaldehyde in the presence of methanol. The use of formaldehyde as areadily available formalin solution introduces water into the reactionwhich may have a deleterious effect on the catalyst used and is likelyto promote the hydrolysis reactions of the methyl propionate reactantand of the methyl methacrylate product. Water is produced as aby-product of the methacrylate synthesis reaction and it is thereforedesirable to reduce to a minimum the amount of water that is introducedinto the reaction zone with the feeds so that the level of water in thereactor is maintained as low as possible. Processes for the productionof methyl methacrylate, in which propionic acid or its methyl ester isreacted with formaldehyde or methylal in the presence of methanol, aredisclosed in U.S. Pat. No. 3,535,371, U.S. Pat. No. 4,336,403,GB-A-1107234, JP-A-63002951. However there is no disclosure in thesereferences of how to prepare the feed materials, particularly theformaldehyde, for the required reaction.

U.S. Pat. No. 4,040,913 discloses the use of water entraining compoundssuch as benzene, toluene or methylisobutyl ketone, and FR-A-2409 975discloses the use of saturated C₄-C₆ hydrocarbons. GB-A-1301533discloses the use of alkanols being C₃-C₆ compounds but the alkanols arenot used as water entraining compounds.

It is therefore an object of the invention to provide a process for thepreparation of a formaldehyde feed product from aformaldehyde-containing solution.

It is a further object of the invention to provide a process fortreatment of a formaldehyde-containing solution to yield a formaldehydefeed suitable for use in a methyl-methacrylate production process whichcontains a reduced amount of water compared to the originalformaldehyde-containing solution.

According to the invention, a process for separating aformaldehyde-containing product from a formaldehyde solution comprisingformaldehyde, water and methanol, wherein said formaldehyde-containingproduct contains substantially less water than said formaldehydesolution, comprising distilling said formaldehyde solution in thepresence of a water entraining compound.

The water entraining compound is selected such that it is capable ofdissolving water, formaldehyde, methanol and hemiformal compounds formedby the reaction of formaldehyde with methanol. The water entrainingcompound is preferably a saturated or unsaturated carboxylic acid orester or a carbonyl compound which is substantially unreactive towardsformaldehyde under the conditions of the distillation and also able toentrain the water, preferably by forming a minimum boiling azeotropicmixture with water Preferably the water entraining compound forms aheterogeneous minimum boiling azeotrope with water. Suitable compoundsinclude C₄-C₈ alkanoic acids and their lower alkyl, e.g. C₁-C₈, esters,and ketones having at least 4 carbon atoms such as diethyl ketone.Particularly preferred compounds are esters and methyl propionate andmethyl methacrylate have been found to be especially useful in certainprocesses. It is greatly preferred to use a compound which is intendedto be introduced into a process in which the dewatered formaldehydeproduct is intended to be used.

One particularly preferred compound for use in dewatering a formalinsolution to produce formaldehyde suitable for use in a process to reactformaldehyde with methyl propionate in the presence of methanol ismethyl propionate. In one preferred form of the invention we thereforeprovide a process for separating a formaldehyde-containing product froma formaldehyde solution comprising formaldehyde, water and optionallymethanol, wherein said formaldehyde-containing product containssubstantially less water than said formalin solution, comprisesdistilling said formaldehyde solution in the presence of methylpropionate.

According to a second aspect of the invention we provide a process forthe production of methyl methacrylate by the reaction of formaldehydewith methyl propionate in the presence of methanol and of a suitablecatalyst, wherein said formaldehyde is produced from a formaldehydesolution by means of distilling said formaldehyde solution in thepresence of methyl propionate. Suitable catalysts are known in the artand include silica catalysts having alkali metal sites.

The process of this aspect of the invention is beneficial, in thatformaldehyde feed is recovered as a complex with methanol in a methylpropionate rich stream and water is removed from the processsubstantially free of organics. Integration of the process for producingformaldehyde of the present invention with a methyl methacrylateproducing process as described has the additional advantage that theoverall energy requirement for the combined process may be reducedcompared with other formaldehyde dewatering methods.

Although we have found that the process of the present invention isparticularly suitable to provide a formaldehyde feedstock for subsequentreaction for methyl methacrylate production, the invention is notlimited to the production of formaldehyde for such a process and may besuitable to produce dewatered formaldehyde for other applications.

The formaldehyde solution is preferably standard formalin which usuallycontains water and formaldehyde in approximately equal proportions,usually with a small quantity of methanol. At least some of theformaldehyde is normally present as various water-formaldehyde ormethanol-formaldehyde adducts. Normally the term “formaldehyde” will beused here to refer to the total formaldehyde, whether present as freeformaldehyde or in the form of such adducts. The composition of formalinsolutions may vary and the process of the invention may be operated fora variety of different formalin compositions.

It is preferred that the formaldehyde solution is pre-mixed withmethanol before the distillation process. The methanol/formalin mixtureis preferably allowed to equilibrate so that formation ofmethanol-formaldehyde adduct species is promoted, e.g., by allowing themixture sufficient time to equilibrate, or by stirring or adjusting thetemperature of the mixture. Preferably a suitable amount of methanol isused to provide a molar ratio of methanol to formaldehyde of 0.3-1.5:1,more preferably 0.5-1.2:1, especially 0.8-1.1:1. The methanol may beprovided in the form of a mixture with methyl propionate, for example toenable recirculated methanol from the distillation or form an associatedprocess or process step to be used.

The amount of water entraining compound introduced into the distillationis in excess of the amount required to form an azeotropic mixture withthe water and preferably also with any methanol present so that thewater in the mixture is more volatile than the formaldehyde adducts.Methyl propionate forms an azeotropic mixture with water comprising 92%methyl propionate and 8% water. The methyl methacrylate/water azeotropecontains about 14% water (by mass) and the diethyl ketone/waterazeotrope contains about 84% diethyl ketone by mass. Preferably therelative proportion of water entraining compound to formaldehyde in thebase of the column is in the range 5:1-20:1, e.g. about 10:1 by mass.However, when the water-entraining compound is refluxed, the amount fedto the column may be adjusted as necessary.

The majority of the water is removed as a mixture with the waterentraining compound. A stream containing most of the water may beconveniently removed from the distillation process as a liquidsidestream, e.g. by using a chimney tray or similar device at anappropriate position in the column. A suitable position for withdrawingsuch a sidestream may be determined by considering the composition ofthe liquid phase throughout the column by known methods in the art.Methyl propionate and water form a heterogeneous azeotrope comprising92% methyl propionate w/w. This azeotropic mixture may be separated in adecanter and the organic phase thus produced, which comprises mainlymethyl propionate may be refluxed into the distillation process. Theaqueous phase produced from the decanter may be further treated in asecond distillation unit, preferably at elevated pressure, to give anaqueous stream which is largely free of methanol, formaldehyde andmethyl propionate. The overheads from the second distillation unit canbe recycled back to the main column or to the decanter or to theformaldehyde solution pre-treatment, if present.

Most of the formaldehyde is taken as a bottom product as a mixture withthe water entraining compound. This mixture may be used directly ifrequired for a further reaction in which both compounds are usedtogether, e.g. in the synthesis of methyl methacrylate. Thisformaldehyde-containing mixture contains substantially less water thanthe formaldehyde solution fed to the process. For example, in a typicalprocess according to the invention using methyl propionate as the waterentrainer, a formalin solution containing formaldehyde and water at aweight ratio of approximately 1:1 may produce a dewatered formaldehydestream containing formaldehyde and water at a weight ratio of about10:1.

The process of the present invention is preferably carried out in such away that the purified formaldehyde product may be used in a furtherprocess for which it is required. Preferably the process of the presentinvention is integrated with such a further process so that the purifiedformaldehyde product is supplied directly to that further process. Whenthe formaldehyde product from the process of the present invention isintended to be used as a feedstock together with the water entrainingcompound in a further process, e.g. for the production of methylmethacrylate from formaldehyde and methyl propionate when methylpropionate is used as the water-entraining compound, then the process ofthe present invention and the manufacture of methyl propionate maybeneficially be operated in proximity to or integrated with the furtherprocess.

In a preferred arrangement, methyl propionate is produced by a processwhich provides a source of methyl propionate mixed with methanol and,optionally, water. This mixture may be fed to the distillation of thepresent invention to extract the formaldehyde from a formalin solution.In this way, the separation of the methyl propionate from the process inwhich it is produced may be avoided. When the feedstocks used containmethanol as described, an azeotropic mixture of methyl propionate andthe methanol not complexed with the formaldehyde may be removed from thetop of the distillation column. That mixture may be recycled or removedto storage or a further process.

When the formaldehyde from the process of the invention is intended tobe reacted with methyl propionate and methanol to produce methylmethacrylate, then the process of the present invention is especiallyconvenient, particularly if used in conjunction with a methylpropionate-producing process as described above. Thus, in a secondpreferred arrangement, the dewatered formaldehyde mixture with methylpropionate is used, directly or after an intermediate treatment, in sucha methyl methacrylate process. Similarly, a methyl propionate—methanolstream, which may be derived from a process to produce methylpropionate, and used in the present invention may also be used to makemethyl methacrylate in an integrated process.

One embodiment of the invention will now be further described, by way ofexample only, with reference to the accompanying drawings FIG. 1, whichis a schematic process diagram.

Formalin solution is mixed with methanol and introduced intodistillation column 10 as feed 22. Distillation unit 10 includes achimney tray facility to withdraw some or all of the liquid at alocation above feed 22. Above the chimney tray an additional feed, 21,is admitted which includes methanol, water and methyl propionate whichmay have been derived from a previous process. A sidestream, 40, isremoved at the chimney tray which may represent the whole of the liquidflow in the column and phase separated in decanter 12. The organic phaseis returned to the column below the chimney tray as stream 43 whilst theaqueous phase is passed to column 11 for further processing as stream41. Column 10 is typically operated such that the majority of theformaldehyde exits the bottom as stream 31 complexed with methanol, freemethanol is removed overhead as its azeotrope with methyl propionate asstream 30 whilst water is removed via the sidestream and decantersystem.

Column 11 is operated to drive formaldehyde, methanol and methylpropionate overhead in stream 42 with some water. This is best achievedat elevated pressures typically between 3 and 20 bar. The bottomsproduct is a clean water stream 31 which can either be reused in theprocess or removed. Stream 42 is show as being returned to decanter 12but it could be returned directly to column 11 or mixed with stream 22and hence recirculated to column 10.

EXAMPLE 1

Formalin consisting of 28.5% formaldehyde, 30.7% methanol and 40.8%water by mass was mixed with methyl propionate such that methylpropionate represented 43.5% by mass of the mixture. This was allowed tostand for several hours so that the mixture equilibrated and was thenfed to plate 15 of a 40 plated Oldershaw column, numbering from thebottom upwards, at a rate of 150 ml/h. The column was equipped with awater cooled condenser and a decanter. Pure methyl propionate was addedto the decanter of a rate of 360 ml/hr. The organic phase from thedecanter was refluxed to the column whilst the aqueous phase wascollected and analysed.

90% of the formaldehyde fed was recovered in the stream taken from thebottom of the column and 95% of the water fed was recovered in theaqueous overhead stream. The bottoms stream contained formaldehyde:waterin the ratio of 1:0.076 whereas the original formalin containedformaldehyde to water in the ratio of 1:1.4.

EXAMPLE 2

In an example of operating the process of the invention on a continuousbasis, formalin solution consisting of 55% water, 35% formaldehyde and10% methanol was mixed with an azeotropic mixture of methyl propionateand methanol resulting in a mixture containing, by mass, 20% methylpropionate, 23% methanol, 35% water and 22% formaldehyde. The mixturewas allowed to stand for at least 12 hours to allow the formaldehydeadducts to equilibrate. The mixture was then fed to stage 30 (countingfrom the bottom stage) of a 100 stage Oldershaw column at a rate of 18ml per hour. A second feed containing 86% methyl propionate, 9%methanol, 3% water and 2% formaldehyde was fed to stage 80 of the columnat a rate of 162 ml per hour. A sidestream containing the total columnliquid flow was taken from stage 60 and fed to a water-cooled decanterwhere it was allowed to phase separate. The organic phase was returnedto the column as a reflux and the aqueous phase was removed.

After operating the column continuously for 12 hours, the total productscollected from the top and bottom streams from the column were analysed.The top product from the column was an azeotropic mixture of methanoland methyl propionate. The bottom product from the column containedapprox. 0.3% water, 4.75% formaldehyde with the balance being methylpropionate. Therefore the mass ratio of water to formaldehyde had beenreduced from 1.57:1 in the formalin solution to 0.06:1 in the bottomcolumn product stream.

What is claimed is:
 1. A process for separating aformaldehyde-containing product from a formaldehyde solution comprisingformaldehyde, water and methanol, wherein said formaldehyde containingproduct contains substantially less water than said formaldehydesolution, comprising distilling said formaldehyde solution in thepresence of a water entraining compound wherein said formaldehydesolution contains methanol at a molar ratio of methanol to formaldehydeof 0.3-1.5:1.
 2. A process as claimed in claim 1, wherein theformaldehyde solution is pre-mixed with a quantity of methanol beforethe distillation process such that the molar ratio of methanol toformaldehyde is in the range of 0.3-1.5:1.
 3. A process as claimed inclaim 1, wherein the water entraining compound is selected from thegroup consisting of a saturated or unsaturated carboxylic acid, an esterand a carbonyl compound.
 4. A process as claimed in claim 3, wherein thewater entraining compound is methyl propionate or methyl methacrylate.5. A process as claimed in claim 1, wherein the distilling is carriedout in a column wherein the ratio of water entraining compound toformaldehyde in the base of the column is in the range of 5:1-20:1 bymass.
 6. A process as claimed in claim 1, wherein a liquid sidestreamcontaining at lest the majority of the water contained in saidformaldehyde solution is withdrawn from the distillation process.
 7. Aprocess as claimed in claim 1, wherein said process is integrated with afurther process so that the formaldehyde-containing product is supplieddirectly to said further process.
 8. A process as claimed in claim 7,wherein said further process is a process for the production of methylmethacrylate.
 9. A process for the production of methyl methacrylate bythe reaction of formaldehyde with methyl propionate in the presence ofmethanol and of a suitable catalyst, wherein said formaldehyde comprisesa formaldehyde-containing product produced from a formaldehyde solutionby means of distilling said formaldehyde solution in the presence ofmethyl propionate, wherein said formaldehyde solution contains methanolat a molar ratio of methanol to formaldehyde of 0.3-1.5:1 and whereinthe formaldehyde-containing product contains substantially less waterthan said formaldehyde solution.